Ischemic stroke is one of the major causes of mortality and disability in this country despite advances in the treatment of risk factors contributing to stroke. A host of new therapies designed to intervene in the first few hours of ischemic symptoms have undergone animal testing and are now undergoing controlled clinical trials. Newly developed MRI techniques such as diffusion-weighted imaging (DWI), perfusion imaging, and phase contrast (PC) MR can give critical physiologic information during the acute stages of ischemic stroke. These pulse sequences provide information about the state of injury at the cellular level, perfusion at the capillary or tissue level and blood flow at the level of major cerebral arteries thereby providing many pieces of key physiologic information with one imaging modality. These MRI techniques have largely been evaluated in animal models of ischemic disease and they now need to be evaluated in clinical the setting of acute stroke in humans. This study will evaluate 45 patients with acute carotid territory strokes within 6 hours of symptom onset using these pulse sequences. Imaging will be repeated 3-6 hours, 24- 36 hours, and 5-7 days following symptom onset and 1 month later. The volume of ischemic tissue imaged at the first time point will be determined by DWI and perfusion imaging, and blood flow in the middle cerebral artery (MCA) will be measured with PC MR. These data will be compared to later time points and to the final volumes of infarcted territory as determined by T2-weighted spin-echo imaging at 1 month. Acute imaging results will also be correlated with neurologic examination at presentation, 24 hours and 1 month later. The longitudinal data will be used to characterize the time-related changes following acute stroke with these MR techniques. This study will establish that these MRI techniques provide sensitive measures and prognostic indicators of infarction and ischemia in the early stages of acute stroke. We will demonstrate that this combination of MR imaging protocols constitutes a single imaging modality that is optimal for ischemic stroke diagnosis and is capable of monitoring new therapies such as cytoprotective and thrombolytic agents for the treatment of acute ischemic stroke.